Problem is, the average user will never ever do that or even think it's possible.
Everyone else will bypass protections and continue as before.
https://www.zdnet.com/article/illicit-certificates-worth-mor...
I think the security effect will be marginal.
Is this Apple's position, or is their position that apps you install on your system should be vetted in some way (either via the App Store, or via the notarization process)? It seems to be the latter based on reading the link, but I might be wrong.
There are various ways around this if you know what you're doing.